Preparation of TiO 2 -coated Hydroxyapatite Single Crystals
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TiO2-coated hydroxyapatite single crystals were prepared by hydrolysis of Ti-bearing dicalcium phosphate powder. On the surface of the TiO2-coated hydroxyapatite single crystals, TiO2 regions and hydroxyapatite regions can function as photocatalysts and adsorbents for organic contaminants, respectively, allowing simultaneous decomposition of organic contaminants by the photocatalysis of TiO2 under irradiation. Therefore, the TiO2-coated hydroxyapatite single crystals can favorably be applied for antibacterial applications and environmental purification. Titanium dioxide (TiO2) is of interest for antibacterial applications and environmental purification due to its superior photocatalytic properties and nontoxicity.1 Even a faint light can excite TiO2 enough to decompose organic contaminants in water and air. However, this process requires objects to keep direct contact with TiO2 until the completion of the decomposition, because the photocatalysis is effective only on the surface of TiO2. In practice, TiO2 is required to be combined with nonpolluting adsorbents. Hexagonal hydroxyapatite (HA) is a common adsorbent for NOx and proteins due to its positively charged C-site and negatively charged Psite on the surface. 2 HA is distributed widely in nature and hence nonpolluting. Therefore, HA–TiO2 composites were worth attention to compensate capability of TiO2 for the antibacterial applications and environmental purification with less damage on environment. TiO2–HA composites should have alternatively ordered TiO2 and HA regions to allow subsequent decomposition of objects adsorbed on HA surface. Recently, highly ordered TiO2–HA composites were addressed using biomimetic process such as HA formation on the surface of TiO2 thin film3 and coprecipitation such as modification of HA with Ti(IV).4 This study presents a new solution to establish a highly ordered TiO2–HA composite by coating TiO2 on the a face of HA single crystals. TiO2-coated HA single crystals were prepared by hydrolysis of Ti-bearing dicalcium phosphate (Ti-DCP) powder. The Ti-DCP powder was prepared by adding four-nine-grade calcium carbonate powder (Ube Materials Industries, Japan) up to saturation with vigorous stirring into a 5 mol/l orthophosphoric acid solution (Wako, Japan) containing 0.035 mol/l Ti(III) at room tempera-
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Address all correspondence to this author. J. Mater. Res., Vol. 15, No. 6, Jun 2000
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ture, followed by heating up to 100 °C at a heating rate of 1.4 °C/min by a drying oven. The Ti-DCP powder was separated from the solution by suction, then rinsed 5 times with ultrapure water to remove the excess orthophosphoric acid and water-soluble calcium phosphates. The Ti-DCP powder (1 g) and ultrapure water (60 cm3, electric resistance ⳱ 18.3 M⍀/cm) were packed into a stainless steel pressure vessel with a Teflon inner vessel, and hydrolyzed with stirring at 523 K using an electric furnace for 3 weeks. The hydrolyzed Ti-DCP powder was recovered after quenching. The products were ide
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